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Shows how a PlaceholderStrategy can be used to keep the layout more persistent. If a Dockable is closed and later
made visible again, then it keeps its location.
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One of the more interesting features: stack two Dockables. Close one of them and move the other around. If you now
open the closed Dockable again, then the stack will reapear. Without placeholders the closed Dockable would appear
at its former position.
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